Significant improvements in animal performance, efficiency and carcass and meat quality have been made over the years through the application of standard animal breeding and selection techniques. However, such classical animal breeding techniques require several years of genetic evaluation of performance records on individual animals and their relatives and are therefore very expensive. Other efforts have been made to improve productivity and quality through the application of such management practices as the use of feed additives, animal hormonal implants and chemotherapeutics. However, there is significant political and regulatory resistance to the introduction and use of such methodologies. Such methodologies are also non-inheritable and need to be applied differently in every production system.
There is a need for methods that allow relatively easy and more efficient selection and breeding of farm animals with an advantage for an inheritable trait of circulating leptin levels, feed intake, growth rate, body weight, carcass merit and carcass composition. The economic significance of the use of genetic markers that are associated with specific economically important traits (especially traits with low heritability) in livestock through marker-assisted selection cannot therefore be over-emphasized.
Leptin, the hormone product of the ob (obese) gene, has been shown to be predominantly synthesized and expressed in adipose tissues (Zhang, Y., R. Proenca, M. Maffei, M. Barone, L. Leopold, and J. M. Friedman (1994) Positional cloning of the mouse obesity gene and its human homologue, Nature 372: 425-432; Ji, S., G. M. Willis, R. R. Scott, and M. E. Spurlock. 1998. Partial cloning and expression of the bovine leptin gene, Anim. Biotechnol. 9: 1-4). It functions as a potent physiological signal in the regulation of body weight, energy expenditure, feed intake, adiposity, fertility and immune functions (Houseknecht, K. L., Baile, C. A., Matteri, R. L. and Spurlock, M. E. (1998) The biology of leptin: a review. Journal of Animal Science 76: 1405-1420; Lord, G. M., G. Matarese, J. K. Howard, R. J. Baker, S. R. Bloom, and R. I. Lechler (1998) Leptin modulates the T-cell immune response and reverses starvation-unduced immunosuppression. Nature 394: 897-900; Garcia, M. R., M. Amstalden, S. W. Williams, R. L. Stanko, C. D. Morrison, D. H. Keisler, S. E. Nizielski, and G. L. Williams (2002) Serum leptin and its adipose gene expression during pubertal development, the estrous cycle, and different seasons in cattle. J. Anim. Sci. 80: 2158-2167). Leptin has been proposed as one of the major control factors contributing to the phenotypic and genetic variation in the performance and efficiency of cattle.
Polymorphisms in the coding regions of the leptin gene in cattle have been associated with milk yield and composition (Liefers, S. C., te Pas, M. F. W., Veerkamp, R. F. and van der Lende, T. (2002) Associations between leptin gene polymorphisms and production, liveweight, energy balance, feed intake and fertility in Holstein heifers. Journal of Dairy Science 85:1633-1638), feed intake (Liefers et al., 2002; Lagonigro, R., P. Wiener, F. Pilla, J. A. Woolliams, and J. L. Williams. 2003. A new mutation in the coding region of the bovine leptin gene associated with feed intake. Anim. Genet. 34: 371-374), and body fat (Buchanan, F. C., Fitzsimmons, C. J., Van Kessel, A. G., Thue, T. D., Winkelman-Sim, C. and Schmutz, S. M. (2002) Association of a missense mutation in the bovine leptin gene with carcass fat content and leptin mRNA levels, Genet. Sel. Evol. 34: 105-116; Lagonigro et al., (2003)). However, it would appear that polymorphisms located in the promoter region of the leptin gene (i.e. the region of the gene that regulates the level of leptin expression through its associated enhancer and silencer elements) may have a stronger effect on the regulation of these economically important traits, and therefore be of greater predictive value.
In the present invention it has surprisingly been shown that three previously unknown single nucleotide polymorphisms (SNPs) in the promoter region of the leptin gene, and one previously known SNP in exon 2 of the leptin gene, are strongly associated with several of these economically important traits in cattle. To the best of our knowledge, the genetic markers of the present invention are the first to be identified that show a direct relationship with body weight and feed intake.